1. Field of the Invention
The present invention relates to a flyback transformer. More specifically, the present invention relates to a flyback transformer including a voltage detecting winding for detecting a voltage for the purpose of restricting an x-ray amount generated from a cathode ray tube in a television receiver.
2. Description of the Prior Art
FIG. 1 is a schematic diagram of one example of a circuit for restricting an x-ray amount generated from a cathode ray tube in a television receiver, in which the present invention can be advantageously employed. A flyback transformer 10 comprises a low voltage coil 11, a high voltage coil 12 and a voltage detecting coil 13. A resonance capacitor 111 is connected to the low voltage coil 11 for the purpose of harmonic tuning. The low voltage coil 11 is shunted with a damper diode 112 and is also shunted with a series connection of a horizontal deflection coil 113 and a capacitor 114. The high voltage coil 12 is divided into two, for example, the number of which may be increased as necessary, and a rectifying diode 123 is interposed in series between the divided high voltage windings 121 and 122, and a rectifying diode 124 is connected to the other divided high voltage coil 122. The output of the rectifying diode 124 is connected to a high voltage terminal 125. The high voltage coil 12(121) is connected to the base electrode of a transistor 21 constituting a protecting circuit 20. The output of the voltage detecting coil 13 is rectified by a rectifying diode 131 and is smoothed by a smoothing capacitor 132 and the output thus obtained is applied to one input of a bleeder resistor 23 constituting the protecting circuit 20. The voltage obtained from the bleeder resistor 23 is applied to the emitter electrode of the above described transistor 21 through a Zener diode 24. The base electrode of the transistor 21 is connected through a resistor 22 to a voltage supply +B and further through a resistor to a beam current limiting circuit 30. The beam current limiting circuit 30 is intended to limit the beam current not to exceed a predetermined value. The base electrode of the transistor 21 is further connected to another bias resistor. The output at the collector electrode of the transistor 21 is converted into a direct current voltage by means of an integration circuit 25 and is applied to a horizontal oscillation circuit 40. The horizontal oscillation circuit 40 is structured to be responsive to a predetermined value, say 0.5 V to 0.7 V, of the output voltage obtained from the circuit 25 to stop an oscillating operation. Such a protecting circuit for restricting an x-ray amount generated by a cathode ray tube is well known to those skilled in the art.
In such a conventional television receiver, it is necessary to obtain a voltage or a current associated with the flyback pulse in some form from the flyback transformer 10. Although it is possible to obtain such a current with ease from the high voltage coil 12 of the flyback transformer 10, it is necessary to provide a voltage detecting coil as shown in FIG. 1 in order to obtain such a voltage. As an alternative approach to obtain such a voltage from the flyback transformer 10, an approach may be considered to divide the high voltage developed at the high voltage terminal 125 of the flyback transformer 10 directly by means of a voltage dividing resistor. However, in such a case a resistor of high resistance for voltage dividing a high voltage is required, which not only makes a cost expensive but also causes diversified resistance characteristics, uncertainty by virtue of environmental influences and the like. Therefore, it has been a conventional practice to provide a voltage detecting coil 13 as an expedient for voltage detection, which is most typically coupled to the low voltage coil 13 as a third coil.
The following disadvantages are encountered by such a conventional flyback transformer wherein a voltage detecting coil is electromagnetically coupled to a low voltage coil. More specifically, in general a high voltage output (HV) generated by the high voltage coil 12 is liable to fluctuate in association with the beam current (I), but the conventional flyback transformer involves a problem that the detected output (DV) cannot sufficiently follow a fluctuation of the high voltage output (HV). More specifically, as shown by the curve A in FIG. 2, the conventional flyback transformer involved a problem that a variation of the ratio DV/HV of the detected output (DV) to the high voltage output (HV) with respect to the beam current (I) was conspicuously large. Thus, large fluctuation of the voltage ratio (DV/HV) with respect to a variation of the beam current (I) means that a fluctuation of the high voltage output (HV) and a fluctuation of the detected output (DV) do not occur at the same rate. In such a case, by way of an extreme situation, the protecting circuit 20 is not operable in spite of the fact that the high voltage output (HV) has reached a value in which an x-ray amount is to be restricted, with the result that an x-ray exceeding a predetermined value is generated from the cathode ray tube, not shown. Another problem is that although conventionally a resonance capacitor 111 has been connected to the low voltage coil 11 for the purpose of stabilization of the high voltage output the resonance capacitor is susceptible to a decreasing change of a capacitance value as a function of the lapse of time. In such a situation, assuming that the capacitance of the resonance capacitor 111 has decreased to a half of the original value, the characteristic of the voltage ratio (DV/HV) with respect to the beam current (I) becomes as shown by the curve A' in FIG. 2. Accordingly, in case of a flyback transformer wherein a harmonic tuning has been achieved by means of the low voltage coil 11 and the capacitor 111, the above described disadvantage is more aggravated.